Method for control of a diagnosis of a catalyst in the exhaust of an internal combustion engine

ABSTRACT

A control for diagnosing a catalytic converter in the exhaust gas of an internal combustion engine having means for determining the catalytic converter temperature is presented. The diagnostic method is carried out in dependence upon the catalytic converter temperature. In the diagnosis, only such results are used which have been determined below a threshold value for the catalytic converter temperature.

RELATED APPLICATIONS

This application is the national stage of PCT/DE 01/04920, filed Dec.22, 2001, designating the United States and claiming priority fromGerman patent application no. 100 65 123.2, filed Dec. 28, 2000, theentire content of which is incorporated herein by reference.

FIELD OF THE INVENTION

The invention relates to a method for diagnosing catalytic converterswhich are used to convert toxic substances in the exhaust gas ofinternal combustion engines.

BACKGROUND OF THE INVENTION

Statutory requirements provide an on-board diagnosis of toxic-emissionrelevant vehicle components such as catalytic converters.

Such an on-board diagnosis is known, for example, from U.S. Pat. No.5,987,883. A correction of the conversion capability, which isdetermined with on-board means, for considering temperature influenceson the diagnostic result is known from U.S. Pat. No. 5,526,643. Here, adiagnosis takes place only above a minimum temperature threshold.

There is further an interest to increase the quality and therewith thereliability of the diagnostic statements.

SUMMARY OF THE INVENTION

The subject matter of the present invention increases the reliability ofthe diagnostic statements in that only such diagnostic results areevaluated where the temperature of the catalytic converter was below ahighest temperature threshold for the determination of these diagnosticresults.

This technical teaching is based on the observation that a deterioratedcatalytic converter can effect the same toxic substance conversion atvery high temperatures of, for example, 800° C. as can a new catalyticconverter at 400° C. By limiting the diagnosis to catalytic convertertemperatures below a highest temperature threshold, the situation isprecluded that a deteriorated catalytic converter is evaluated as beingoperational based only on a high diagnostic temperature.

An advantageous configuration results in that, during the drop of thecatalytic converter temperature below the highest temperature threshold,a waiting time must elapse before a catalytic converter diagnosis takesplace or before results are determined which are evaluated in adiagnosis. This waiting time can be dependent upon the speed with whichthe catalytic converter temperature drops. The waiting time for higherrates of change of the catalytic converter temperature are selectedlonger than for lower rates of change.

In this way, the especially significant advantage results that adiagnosis for a steep drop of the catalytic converter temperature from avery high level is delayed longer than in a flat temperature course. Thelonger delay with a steep course allows the catalytic converter moretime for reaching a thermal steady state condition in the diagnostictemperature range between lowest and highest temperatures. For a flattemperature course, a long wait is not required and the diagnosis canbegin earlier.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described with reference to the drawingswherein:

FIG. 1 shows the technical background of the invention.

FIG. 2 shows a flowchart as an embodiment of the method of the inventionand FIG. 3 shows possible courses of the catalytic converter temperatureas a function of time.

DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

FIG. 1 shows in detail an internal combustion engine 1 having an intakemanifold 2, load detecting means 3, fuel metering means 4, an rpm sensor5, an exhaust-gas system 6, a catalytic converter 7, exhaust-gas probes8 and 9, a control apparatus 10 and means 11 for displaying a fault.

The control apparatus 10 receives the signals ML of the load detectingmeans, (n) of the rpm sensor and the signals USVK and USHK of the twoexhaust-gas probes and forms therefrom, inter alia, a fuel-meteringsignal ti, for example, an injection pulse width for driving injectionvalves 4 as fuel-metering means. For this purpose, a base value t1 ofthe drive signal ti is determined for the fuel-metering means as afunction of the inducted air mass ML and the engine speed (n). This basevalue is multiplicatively corrected in a closed control loop by acontrol actuating quantity FR. This control actuating quantity FR isgenerated in a manner known per se by applying a PI control strategy tothe deviation of the probe signal USVK from a desired value. In theformation of the fuel-metering signal, the signal USHK can be includedin the computation additionally, for example, for forming the desiredvalue. A temperature sensor 12 supplies a signal as to the catalyticconverter temperature TKAT to the control apparatus. Alternatively, thecatalytic converter temperature TKAT can also be determined in thecontrol apparatus 10 from other measurement quantities, especially, fromthe air quantity ML and the rpm (n) utilizing a computer model. One suchtemperature modeling is, for example, disclosed in U.S. Pat. No.5,590,521.

In this technical background, the method of the invention can be carriedin the embodiment shown in FIG. 2.

Step 2.1 in FIG. 2 a is reached from a higher-order engine controlprogram and serves for determining the catalytic converter temperatureTKAT. In step 2.2, a comparison of the catalytic converter temperatureTKAT to a predetermined maximum value TMAX takes place. If theTMAX-value is exceeded, then no diagnosis takes place and the mainprogram is continued. If, in contrast, the catalytic convertertemperature TKAT remains below the threshold value TMAX, then thediagnosis is permitted in step 2.3 and the main program is continuedafter the diagnosis.

In this way, only such results are used for diagnosis where theseresults were determined with the catalytic converter temperature lyingbelow the threshold value TMAX.

When the diagnosis, for example, is based on a quotient of the signalsof the exhaust-gas probes forward and rearward of the catalyticconverter, then this can mean that the quotient is formed only for apermitted diagnosis. Alternatively, the quotient can be formedcontinuously but the quotient can be evaluated only for permitteddiagnoses.

This distinction applies in the same manner for other diagnosticmethods.

FIG. 2 b shows a supplement of FIG. 2 a as an additional embodiment.According to this embodiment, the determination of a waiting time tvcomes after step 2.2 in a step 2.2.1 and the initialization of a countvariable t (t=0) is made in step 2.2.2. The waiting time tv can be afixed value. Alternatively, tv can be dependent upon operatingparameters of the engine or the exhaust-gas system. A dependency of thewaiting time tv on the speed of the change of the catalytic convertertemperature when there is a drop below TMAX is especially advantageous.The dependency tv=F(d/dt(TKAT)) is to be so designed that the waitingtime tv is that much longer the steeper the course of TKAT is whenpassing the threshold value TMAX. The steps 2.13 and 2.14 form a waitingloop which is run through so often until the waiting time has elapsed.In this case, the program is continued with step 2.3 and the diagnosisis thereby enabled.

FIG. 3 shows the various waiting times in dependence upon the timederivative of the temperature TKAT when there is a drop below thethreshold value TMAX.

1. A method for controlling the diagnosis of a catalytic converter inthe exhaust gas of an internal combustion engine having means fordetermining the temperature of said catalytic converter, the methodcomprising the steps of: carrying out the diagnosis in dependence uponsaid temperature of said catalytic converter; and, utilizing only suchresults for said diagnosis which were determined below a threshold value(TMAX) for said temperature of said catalytic converter and for whichresults said temperature of said catalytic converter was below saidthreshold value (TMAX) for a predetermined waiting time.
 2. The methodof claim 1, wherein said temperature of said catalytic converter mustlie within a temperature interval (TMAX, TMIN).
 3. The method of claim1, wherein the length of the predetermined waiting time is dependentupon the speed with which the catalytic converter temperature drops. 4.A method for controlling the diagnosis of a catalytic converter in theexhaust gas of an internal combustion engine having means fordetermining the temperature of said catalytic converter, the methodcomprising the steps of: carrying out the diagnosis in dependence uponsaid temperature of said catalytic converter: utilizing only suchresults for said diagnosis which were determined below a threshold value(TMAX) for said temnerature of said catalytic converter and for whichresults said temperature of said catalytic converter was below saidthreshold value (TMAX) for a predetermined waiting time; wherein thelength of the predetermined waiting time is dependent upon the speedwith which the catalytic converter temperature drops; and, thepredetermined waiting time for higher temperature rates of change isselected longer than for lower temperature rates of change.
 5. Anarrangement for diagnosing a catalytic converter in the exhaust gas ofan internal combustion engine, the arrangement comprising: means fordetermining the temperature of said catalytic converter; means forcomparing said temperature of said catalytic converter to apredetermined threshold value (TMAX); decision means for utilizing onlysuch results for said diagnosis which were determined below saidthreshold value (TMAX) for said temperature of said catalytic converter;and, said comparing means functioning to use only said results for whichsaid temperature of said catalytic converter was below said thresholdvalue (TMAX) for a predetermined waiting time.